Closely-coupled multiple-winding magnetic induction-type sensor

ABSTRACT

The invention relates to sensors for detecting a time-varying current by employing a plurality of coils with separate windings, disposed on separate toroidal cores, that are placed in close proximity of each other so that each coil responds independently to the same current. The current produces a time-varying magnetic field which in turn induces a plurality of voltages across the coils that can be combined to provide a resultant signal. Some embodiments of the invention employ coils with opposite windings to obtain signals with opposite phases and combine these signals through differential detection means to obtain a combined signal. One aspect of the invention relates to production of a wide-band or a selectable band-width sensor by preparing at least one coil to be dissimilar with respect to the others. In addition, the invention provides provisions for easy calibration of the sensor.

BACKGROUND

This invention relates to methods and apparatus for detecting atime-varying current. In particular, the invention detects a transientcurrent produced by a variety of physical mechanisms, e.g., a pulse ofcurrent carried on an electrical conductor, with a bettersignal-to-noise ratio than the existing current sensors provide.

One important application of the apparatus of the invention relates tothe detection of a sudden localized redistribution of charge at adefective point of an insulation which is under high electric stress,e.g., electric power cables, high voltage transformers. Such aredistribution of charge leads to an intermittent arcing that signalsthe presence of a fault. The term fault, as used herein, refers both toan incipient fault which may not cause an immediate failure but mayeventually lead to a failure, and also to a complete failure of thesystem. It is well-known that partial discharge events are commonprecursors to many significant failure modes in a variety of highvoltage devices.

The early detection of an incipient fault through monitoring partialdischarge events can prevent the occurrence of a complete failure whichmay result in considerable inconvenience and financial loss. Forexample, the failure of high voltage devices of a utility company suchas transformers and high voltage distribution cables, especially duringa critical period, can lead to enormous inconvenience for the customersand financial loss for the company.

The energy associated with a partial discharge event is typicallyextremely small. This energy is particularly small when the defect thatgives rise to the partial discharge is at its early stages ofdevelopment. Given the desirability of detecting defects at such earlystages, it is evident that having a sensor that detects a pulse ofcurrent with a high signal-to-noise ratio is extremely desirable.

U.S. Pat. No. 5,075,629 discloses a device for detecting a partialdischarge in a transformer. In particular, this patent discloses anantenna, constructed of a single solid core with a conducting wirewrapped around it, that is placed inside the transformer to detectemission of an electromagnetic wave produced by the transformer as aresult of the occurrence of a partial discharge. The center of theconducting wire wrapped around the single core is grounded to producetwo coils. The time-varying magnetic field of the electromagnetic waveinduces two voltages in the coils that provide the input signals for adifferential amplifier. The output of the amplifier signals theoccurrence of a partial discharge.

The antenna disclosed in the '629 patent has a solid core that does notpermit the passage of a cable therethrough. Thus, the apparatus can notbe connected directly through a cable to the transformer to receive acurrent pulse that a partial discharge produces. Accordingly, it relieson a weaker mode of coupling, i.e., detection of the electromagneticwave emanated from the transformer. This mode of coupling imposes thefurther limitation that the apparatus can not be employed outside thetransformer because high voltage transformers are typically shielded byenclosures.

Some prior art techniques relate to finding the location of a partialdischarge event so as to allow correction of the defect giving rise tothe discharge. For example, U.S. Pat. No. 5,530,364 discloses anapparatus for detecting the location of an incipient fault in aninsulated cable. In particular, the apparatus of the invention scans thesurface of the cable by physically moving two separate axially spacedsensors, disposed adjacent to the surface, over the cable. A partialdischarge event produces a current pulse through the cable which, inturn, produces electrical pulses at each of the two sensors. These twopulses are added to produce a resultant signal whose magnitude reaches amaximum if the partial discharge occurs at a point midway between thetwo sensors, thus indicating the location of the discharge.

The energy produced by a partial discharge event in a high voltagedevice is typically extremely small. Accordingly, it is imperative thatthe system designed for detecting such events provide a means ofcoupling to the device that has a minimal loss and also a detectionmeans that provide a high signal-to-noise ratio. In addition, periodicmonitoring of a device for partial discharges requires a system that canbe easily connected to and disconnected from the device. Many prior artsystems suffer from a number of limitations related to theabove-mentioned desirable features that the present invention seeks toremedy. For example, the use of the apparatus of '629 patent not onlyemploys a weak mode of coupling to the transformer but also requiresopening up a transformer to place the antenna within it. The apparatusof the '364 patent is essentially a timing system for signaling thearrival of two pulses at the sensors, and does not provide newtechniques for improving the signal-to-noise ratio of the detectedpulses.

It should be understood that the desirability of detecting transientcurrents with a high signal-to-noise ratio is not limited to monitoringof partial discharges. Another possible application, for example,relates to monitoring a beam of electrons or ions used in implantationsystems.

Accordingly, it is an object of the present invention to detect atransient current carried by a wire with an improved signal-to-noiseratio.

It is another object of the invention to detect a pulse of current or abeam of charged particles with an improved signal-to-noise ratio.

It is yet another object of the invention to provide a current sensorthat can be easily calibrated.

It is yet another object of the invention to provide a current sensorwith a broad-band and/or selectable frequency response.

It is yet another object of the invention to provide an apparatus formonitoring partial discharges in high voltage devices with an improvedsignal-to-noise ratio.

The invention is next described in connection with illustratedembodiments. It is obvious to those skilled in the art that variousmodifications to the embodiment can be made without departing from thescope and the spirit of the invention.

SUMMARY OF THE INVENTION

The invention detects a time-varying current by employing a plurality ofgenerally toroidal coils with separate windings that are placed in closeproximity of each other such that they respond to a current signal,which passes through the central aperture of the toroids, atsubstantially the same time. In particular, the current induces aplurality of voltages across these coils where each induced voltageindicates the detection of the current. The invention also allowscombining the induced voltages, i.e., summing and/or subtracting thevoltages, to produce a resultant signal that signifies the existence ofthe time-varying current. A single coil of toroidal construction issometimes referred to as a Rogowski coil.

It is well understood in physics that a changing magnetic flux within acoil induces a voltage across it. Furthermore, a current due to movingcharged particles produces a surrounding magnetic field. Accordingly, achanging magnetic flux, associated with a current pulse, through theclosely-spaced coils of the present invention induces a plurality ofvoltages across them. The spacings between the coils are selected suchthat each coil responds to substantially the same magnetic field. Inaddition, each coil is electrically insulated from the others so thateach induced voltage represents an independent response to the samecurrent. The induced voltages can be either utilized individually or canbe combined in various ways, described more fully below, to produce acombined signal.

One aspect of the invention relates to selecting at least one coil tohave either a winding of opposite polarity with respect to another coilor to have a winding of similar polarity but reverse output connectionswith respect to the other coil. Two such coils are herein referred to ashaving opposite polarities with respect to each other. The inducedvoltages across two coils having opposite polarities have oppositephases relative to each other. In addition, the close proximity of twosuch coils ensures that the character of noise on the induced voltageacross one coil is substantially similar to that on the induced voltageacross the other coil. Accordingly, subtraction of the induced voltagesacross two coils of opposite polarities results in addition of the twoinduced voltages and reduction of the noise, i.e., an improved signal tonoise ratio.

The invention also allows selection of at least one coil of theplurality of closely-spaced coils as a calibration coil. The injectionof a known driven current into such a calibration coil induces a currentin an electrical conductor, disposed in the middle of the sensor, whichin turn produces a time-varying magnetic flux in the other coils. Thetime-varying magnetic field induces a plurality of voltages across thesecoils. Because the value of the injected current in the calibration coilis known, the value of the response induced in the other coils by theinjected current can be readily calculated. Thus, the measurement of theinduced voltages due to the injected current provides calibration of thecoils.

In another aspect of the invention, the closely-spaced coils are adaptedfor detection of a time-varying magnetic field that has differentselected frequency regions including a wide frequency bandwidth. Inparticular, at least one coil is chosen to be different from the others,e.g., at least one coil is chosen to have a different number of windingsand/or different core material. The response of such a coil is optimalat a particular frequency which is different from the frequencies atwhich the other coils exhibit their optimal responses. This results in acurrent sensor with a wider bandwidth and/or with simultaneous optimalresponses in different frequency regions. For example, a broad-bandsensor according to the invention can have one coil with a frequencyresponse centered at 60 Hz, and another coil with a frequency responsecentered at 1 MHz. The invention preferably selects the coils such thatthe aggregate response of the coils span the frequency band-width ofinterest of the time-varying magnetic field. Some embodiments of theinvention sum the induced voltages across the coils of such a wide-bandmagnetic sensor to obtain a resultant signal that signifies the presenceof the time-varying current.

One important application of a wide-band current sensor relates to thedetection of a transient current having a short temporal duration andbeing carried by an electrical conductor. This can be understood bynoting that such a pulse of current has many frequency components in itsfrequency bandwidth. The coils can be designed such that each coilresponds optimally to a selected number of these frequency components.Thus, the aggregate response of all of the coils to the transientcurrent is more effective than the response of each individual coil.

It should be understood that production of a current sensor thatincorporates all of the various features described above is within thescope of the present invention. In particular, a sensor having aplurality of coils such that at least two coils yield oppositepolarities with respect to each other, or at least one coil is acalibration coil, or at least one coil has a frequency response that isdifferent from the frequency response of the others incorporates all ofthe above-mentioned features.

The invention also contemplates disposing a plurality of coils on anumber of cores such that each coil responds to a time-varying currentat substantially the same time. According to one aspect of theinvention, the coils are disposed on separate cores. The material of thecores is selected to have a high magnetic permeability, e.g., ferritematerial, or air, so as to allow penetration of a magnetic field intothe body of the cores. The penetration of the magnetic field into thecores results in a changing magnetic flux within the coils disposed onthe cores which in turn induces a plurality of voltages across thecoils. The structures of the cores can be selected, e.g., toroidal, orsubstantially toroidal, to provide sensors suitable for a variety ofapplications. The use of separate cores allows attaining separateindependent signals from each of the plurality of coils in response tothe same magnetic field.

It is typically desirable to select a magnetic core, on which a coil fordetection of a time-varying current is disposed, to be as large aspossible. The advantage of employing a large core can be understood bynoting that as the size of the core increases, it intersects with largernumber of magnetic field lines of a time-varying magnetic field, thusresulting in a larger magnetic flux within the core. The larger magneticflux, in turn, results in a larger induced voltage across the coildisposed on the core, i.e., a better signal-to-noise ratio. Largemagnetic cores are, however, costly and difficult to manufacture. Theemployment of multiple cores by the present invention achieves theadvantages of utilizing a single large core, e.g., bettersignal-to-noise ratio, without its drawbacks, e.g., higher cost anddifficulty of manufacture.

The toroidal and the quasi-toroidal structures are particularly suitedfor detecting a current pulse being carried on an electrical conductor.One important application of such a detection of a current pulse relatesto signaling the occurrence of a fault in a high voltage device, e.g., ahigh voltage transformer or a high voltage electrical cable. Forexample, the occurrence of a partial discharge in such a high voltagedevice produces current pulses that can be detected by a magnetic sensoraccording to the present invention.

While a number of preferred embodiments of the invention employ aplurality of cores, the invention can also be practiced by employing asingle core with a plurality of coils disposed thereon. A first coil iswound around the core, and subsequently successive coils are disposed onthe first coil, each on top of the previous one. In addition, anelectrically insulating material is disposed between any two adjacentcoils to isolate them from each other so that each coil provides anindependent response to a transient magnetic field. The structure of thesingle core on which the coils are disposed can be selected to betoroidal, and quasi-toroidal.

Similar to sensors according to the present invention that comprise aplurality of cores, a sensor with a single core and a multiplicity ofcoils, as explained above, can also incorporate the various aspects ofthe present invention. In particular, production of such a sensor havingat least a pair of coils with opposite polarities, and/or having atleast one calibration coil, and/or having at least one coil that isdifferent from the others, is within the scope of the present invention.

Another aspect of the invention is to employ the multiple closely-spacedcoils in conjunction with a network of capacitors inductors, andresistors to produce a resonant electronic L-C-R circuit. The use ofsuch a resonant circuit enhances currents at selected frequencies, andhence increases the induced voltages, thus rendering the detection ofweak time-varying magnetic fields feasible. As was mentioned previously,one important application of the sensor of the invention is fordetection of partial discharges in high voltage devices. Because such PDevents are typically of short durations, e.g., less than a microsecond,the values of the inductance, the capacitance, and the resistance of aresonant L-C-R circuit employed in a sensor designed for detection ofsuch events are typically selected to produce a resonant frequency inthe range 30 kHz to 5 MHz.

Thus, the invention allows the differential detection of a transientmagnetic field, and attains the aforementioned advantages including abetter signal-to-noise ratio, a selectable bandwidth, easy calibration,and possible reduction in cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cross sectional view of a sensor according to theinvention that has an enclosure to house a plurality of toroidal cores,each of which has a coil, that permit the passage of an electricalconductor through their hollow centers in order to detect a currentpulse carried by the conductor, and the outputs from each coil are madeavailable as needed,

FIG. 2 is a more detailed illustration of the components of anembodiment of the apparatus shown in FIG. 1, wherein a differentialamplifier produces a signal proportional to the difference of twoelectrical voltages with opposite phases that a current pulse inducesacross two toroidal coils with opposite windings but similar outputconnections, for clarity the distance between the two coils isexaggerated and any enclosure is not shown,

FIG. 2A depicts a sensor similar to that shown in FIG. 2 except that thetwo coils have similar windings and reverse output connections, thusproducing two induced voltages with opposite phases that are inputtedinto a differential amplifier,

FIG. 2B is a cross-sectional view of FIG. 2,

FIG. 2C is a cross-sectional view of FIG. 2A,

FIG. 3 shows a sensor according to the invention that employs twoseparate cores having two coils of opposite polarities, and a network ofinductors, capacitors and resistors to differentially detect a transientmagnetic field,

FIG. 4 is a cross-sectional view of an embodiment of the invention thatutilizes a single toroidal core with two coils with opposite windingsdisposed thereon to differentially detect a transient magnetic field,

FIG. 5 illustrates a sensor that provides a selectable band-widthresponse to a transient current by employing two dissimilar coilsdisposed on two separate cores, and

FIG. 6 is an illustration of a magnetic sensor with two separate cores,each having a coil, wherein one of the coils is employed for calibrationof the other.

DETAILED DESCRIPTION

FIG. 1 illustrates a cross sectional view of an apparatus according tothe invention in which a metallic shield enclosure 10 houses a number ofcores 12 on each of which a conductive coil is mounted. The enclosurehas an aperture that permits the passage of an electrical cable 14therethrough. Output connections 16 and 16A provide access to thevoltages induced at the terminals of the conductive coils. Gaps 18,provided in the enclosure, allow penetration of the magnetic fieldproduced by a current carried by the cable into the coils.

FIG. 2 is a more detailed depiction of the cores and the coils of theapparatus of FIG. 1 according to one preferred embodiment, illustratingtwo separate cores, namely core 20 with a coil 22 disposed thereon andhaving terminals 24 and 26, and core 28 with coil 30 disposed thereonand having terminals 32 and 34. The cores have toroidal structures withhollow centers that allow the passage of an electrical conductor 36therethrough. The distance between the two coils in the figure isexaggerated for clarity, and the enclosure is not shown. In practice,the two coils are placed in close proximity of each other such that theyboth detect the same magnetic field.

In one application of this embodiment, for example in monitoring partialdischarges, a pulse of current passes through the conductor and inducestwo voltages across the two coils. The polarity of the winding of onecoil is selected to be opposite of the other. Terminals 26 and 32 aregrounded, and the voltages induced across the other two terminals, i.e.,terminals 24 and 34 have opposite phases relative to each other. Tworesistors 38 and 40 are connected across the output terminals of coils22 and 30, respectively. The values of the resistance of these resistorsare chosen to influence the frequency response of the apparatus. Adifferential amplifier 42 receives the voltages induced at terminals 24and 34 at its input terminals and produces an output voltage that isproportional to the difference of the two input voltages.

FIG. 2A illustrates an embodiment that similar to the embodiment of FIG.2 employs two separate toroidal cores, on each of which a coil isdisposed. Unlike the previous embodiment, however, the polarities of thewindings of the two coils are selected to be the same, but the polarityof the output connections are reversed. For example, if a transientmagnetic field induces a positive voltage at terminal 24A of coil 22A,it will induce a negative voltage at terminal 34A of coil 30A. Adifferential amplifier 42A receives at one input terminal the positivevoltage induced at one terminal of one of the coils, and at its otherinput terminal the negative voltage induced at the opposite terminal ofthe other coil, and produces a voltage proportional to their difference.

FIGS. 2B and 2C are cross-sectional views of the cores and the coils ofthe sensors shown in FIGS. 2 and 2A, respectively, presented toillustrate further that the invention can produce induced voltages withopposite phases in response to the same time-varying magnetic field intwo different manners. In particular, the coils of FIG. 2B are selectedto have opposite windings but similar output connections so that theinduced voltages across them have opposite phases. The sensor of FIG.2C, however, accomplishes the same result by selecting the two coils tohave similar windings but reverse output connections.

FIG. 3 illustrates an embodiment of the invention that utilizes anetwork of inductors, capacitors, and resistors to enhance the voltageinduced across a plurality of coils disposed on separate toroidal coresas a result of the passage of a current pulse through the hollow centerof the toroids. In particular, two coils 44 and 46 are disposed on twocores 48 and 50, respectively. The coils have opposite windings toproduce two voltages with opposite polarities in response to the sametransient magnetic field.

An electrical conductor 52 passes through the hollow cavity at thecenter of the cores and carries a current pulse. In one application ofthis embodiment, the conductor is connected to a high voltage devicewhich creates a current pulse as a result of the occurrence of a partialdischarge. This current pulse produces a magnetic field that penetrateswithin the cores and creates a time-varying flux within each coil, thusinducing a voltage across each coil. Because the two coils have oppositewindings, the induced voltages across the two coils have opposite phasesrelative to each other. Thus, a positive voltage is induced at theun-grounded terminal of one coil and a negative voltage at thecorresponding terminal of the other coil.

Further reference to FIG. 3 shows two identical capacitors 54 and 56,each connected electrically in parallel with one of the coils. Twoidentical resistors 58, and 60 are connected electrically in parallelwith the capacitors 54 and 56, respectively. Further, two identicalinductors 62 and 64 are connected electrically in parallel with theresistors 58 and 60, respectively. The capacitance, the inductance, andthe resistance of the capacitors, resistors and the inductors areselected in a manner known in the art such that the combination of eachcoil with its associated capacitor, resistor, and inductor produces aresonant circuit with a resonant frequency in the range 30 kHz to 5 MHz.The induced voltages in the coils produce two voltages at terminals 66and 68.

Referring again to FIG. 3, a differential amplifier 70 receives theinduced voltages at terminals 66 and 68 at its input terminals, andproduces an output signal proportional to the difference between the twoinput voltages. Because the two input voltages are 180 degrees out ofphase, the magnitude of the output signal is proportional to the sum ofthe magnitude of the two input signals whereas the root mean squaremagnitude of the noise carried on the output signal is lower than thesum of the root mean square magnitude of the noise on each of the twoinput signals. Accordingly, the signal-to-noise ratio of the outputsignal is higher than that of the input signals. The differentialamplifier can be replaced with a balanced center-tapped transformer toachieve the same result.

If the apparatus is utilized for monitoring a partial discharge, awaveform analyzer 72 receives the output signal of the differentialamplifier and analyzes the waveform associated with the current pulsesin search of the fingerprints of a partial discharge event in a mannerwell-known in the art. Once such a signal is found, the waveformanalyzer signals the occurrence of the event.

As was describe previously, the practice of the invention is not limitedto employing a plurality of cores. In particular, FIG. 4 illustrates across-sectional view of an embodiment of the invention that employs asingle core 74 with a toroidal shape. The core has a high magneticpermeability, e.g., a ferrite material, or an air core, to allowpenetration of a magnetic field into the core. A first coil 76 isdisposed directly on the core with two output terminals 78 and 80. Anelectrically insulating material 82, e.g., polyethylene, is disposed onthe first coil, and subsequently a second coil 84 is disposed on thisinsulating material with two output terminals 86 and 88. Terminals 80and 88 are grounded. The insulating material ensures that the coilsrespond independently to the same time-varying magnetic field. Thepolarities of the windings of the two coils are selected to be oppositeto each other. Accordingly, an induced voltage at the terminal 78 of thefirst coil, produced for example by a current passing through aconductor 90, is opposite in phase with respect to the voltage at theterminal 86 of the second coil. The same type of phase relationshipexists between the other two terminals of the two coils. A differentialamplifier 92 receives two induced voltages with opposite phases andproduces an output voltage proportional to their difference. Thus, thisembodiment also allows a differential detection of a time-varyingmagnetic field.

As was described previously, one aspect of the present invention relatesto producing a sensor with a wide-band frequency response and/orfrequency response selected in particular bandwidths. FIG. 5 illustratesa wide-band sensor according to the invention in which a toroidal core94 with a coil 96 disposed on it is placed in close proximity of anothertoroidal core 98 on which a coil 100 is disposed. The two cores areclosely-spaced so that each coil responds to a magnetic signalsubstantially at the same time. The windings of the two coils have thesame polarity, but the two coils are made intentionally dissimilar. Forexample, the two coils can be selected to have different number ofturns. This results in each coil having a bandwidth of response to atransient magnetic field which is different from that of the other.Thus, the sensor as a whole has a bandwidth which is wider than that ofeach coil.

One application of such a sensor is in detecting a short current pulsethat is carried on an electrical conductor. Referring again to FIG. 5,such a pulse of current being carried on a conductor 102, disposed inthe hollow centers of the cores, produces a time-varying magnetic fluxwithin each coil. The time-varying flux in turn induces two voltagesacross the two coils. A summing amplifier 104 adds these two inducedvoltages to produce a resultant voltage. The Fourier transform of thetemporal profile of the pulse comprises of a number of components.Because the two coils are selected to be different, each respondspreferentially to a particular set of these components. Thus, the twocoils jointly respond to the current pulse more efficiently than asensor with two identical coils.

It should be understood that the coils of any of the embodimentsdescribed above can be made dissimilar in order to construct a wide-bandand/or selectable band-width magnetic sensor. Thus, toroidal, andquasi-toroidal structures can be employed to construct a wide-bandsensor of time-varying magnetic fields.

FIG. 6 is yet another embodiment of the invention that illustrates amagnetic sensor with a provision for easy calibration. In particular,reference to FIG. 6 shows two separate toroidal cores, 106 and 108,disposed in close proximity of each other, and having coils, 110 and112, disposed thereon. The coil wound on the core 106 is employed forcalibration purposes. In particular, further reference to FIG. 6illustrates a signal generator 114 that injects a known driven currentinto the calibration coil. The current produces a magnetic field withinthe core which in turn produces a current through a conductor 116disposed in the hollow centers of the two cores. This current produces aknown magnetic flux through the other core that induces a voltage acrossthe coil 112. A measurement of this induced voltage by a voltmeter 118leads to calibration of the sensor.

The above descriptions of the various embodiments should be interpretedas illustrative and not in a limiting sense. It is obvious to thoseskilled in the art that many variations can be made to the embodimentsdescribed above without departing from the scope and the spirit of theinvention. In particular, although many of the described embodimentswere presented with two coils, it should be understood that the practiceof the invention is not limited to employing only two coils. A pluralityof coils with separate windings and disposed on separate cores or on asingle core, according to the methods of the invention, can be employedto effectuate the objectives of the invention.

Thus, the invention attains the objectives set forth above by employinga plurality of closely-spaced coils that respond independently to thesame time-varying magnetic field.

Having described the invention, what is claimed as new and protected byLetter Patent is:
 1. Apparatus for detecting a pulse of current having aselectable frequency bandwidth, said apparatus comprising a plurality ofelectrically conducting coils having separate windings and at least oneof said coils having a frequency response to said pulse that isdifferent from the frequency response of another of said coils, each ofsaid coils being disposed on a core having a toroidal structure with anopen aperture to allow the passage of said current through the openapertures of the toroidal structures, said cores being disposed in closeproximity of each other such that said pulse induces a plurality ofvoltages across said coils.
 2. The apparatus of claim 1, wherein saidcurrent is carried by an electrical conductor that passes through theopen aperture of each torroidal structure.
 3. The apparatus of claim 1,wherein moving charged particles generate said current.
 4. The method ofclaim 3, wherein a partial discharge in a high voltage device producesthe pulse of current.
 5. The apparatus of claim 1, further comprisingmeans for combining said induced voltages to produce a resultant signal,thereby indicating the passage of said pulse of current.
 6. Theapparatus of claim 1, wherein at least one of said coils is acalibration coil.
 7. A method for detecting a pulse of current, saidmethod allowing detection over a selectable frequency bandwidth andcomprising the step of disposing a plurality of electrically conductingcoils having separate windings in close proximity of each other suchthat said coils respond to said pulse at substantially the same time,wherein at least one of said coils has a frequency response to saidpulse that is different from the frequency response of another of saidcoils, whereby said pulse induces a plurality of voltages across saidcoils, wherein each induced voltage indicates the passage of saidcurrent pulse.
 8. The method of claim 7, further comprising the step ofcombining said induced voltages to produce a resultant signal.